White light-emitting diodes (LEDs) exhibit high efficiency, long lifetimes, less environmental impact, absence of mercury, short response times, applicability in final products of various sizes, and many more favorable properties. They are gaining attention as backlight sources for liquid crystal displays, computer notebook monitors, cell phone screens, and in general lighting.
As known to the expert, white LEDs can be obtained by adding a yellow emitting phosphors, such as YAG:Ce, which exhibits an emission peak wavelength around 560 nm, to a blue light emitting LED. The emitted peak wavelength of corresponding blue light emitting LEDs is typically in the range from 450 to 470 nm. Therefore, only a limited number of phosphors can be used in order to obtain white LEDs because the phosphors need to absorb light in the range emitted from the blue LED.
By combining red, green, and blue emitting phosphors with an near UV LED, which typically emits light at a wavelength ranging from 280 to 400 nm, as a primary light source, it is possible to obtain a tri-color white LED with better luminescence strength and superior white color in comparison to the above described white LEDs. Consequently, there is a considerable demand for phosphors excitable at wavelength ranging from 280 nm to 400 nm.
To obtain such white LEDs by using UV-LEDs or near UV-LEDs, typically a red, a green, and a blue emitting phosphor are first mixed in a suitable resin. The resultant gel is then provided on a UV-LED chip or a near UV-LED chip and hardened by UV irradiation, annealing, or similar processes. The phosphor mixture in the resin should be as homogeneously dispersed as possible in order to observe an even, white color, while looking at the chip from all angles. However, it is still difficult to obtain a uniform distribution of the different phosphors in the resin because of their different particle sizes, shapes and/or their density in the resin. Hence, it is advantageous to use less than three phosphors or even only one phosphor. For example, the use of a phosphor having two or more main emission peaks at different wavelengths represents a potential solution of the above-mentioned problem.
In this connection, Sung Hun Lee, Je Hong Park, Se Mo Son, and Jong Su Kima disclose in Appl. Phys. Lett. 2006, 89, 221916, a CaMgSi2O6:Eu2+, Mn2+ phosphor exhibiting three emission bands peaks at around 450 nm, 580 nm, and 680 nm with main peaks at 440 nm and 680 nm. Since the most preferred spectral range for a human eye is between 400 and 650 nm, the emission peak at 680 nm is located on the edge of the visible range. Moreover, a mixture of green-poor CaMgSi2O6:Eu2+, Mn2+ and green-to-yellow emissive (BaSr)2SiO4:Eu2+ is needed in order to achieve white light having correlated color temperatures from 4845 to 9180 K and color rendering indices from 71% to 88%.
However, even by using a mixture of two phosphors, in order to produce white LEDs using UV or near UV-LEDs, it is still difficult to uniformly mix phosphors having different sizes, particle shapes and densities in the resin. Moreover, the phosphors should not be excited by a wavelength located in the visible range. For instance, if the emission spectrum of the green phosphor overlaps with the excitation spectrum of the red phosphor, then color tuning becomes difficult. Additionally, if a mixture of two or more phosphors is used to produce white LEDs using a blue emitting LED as the primary light source, the excitation wavelength of each phosphor should efficiently overlap with the blue emission wavelength of the LED.
An example of a white LED using UV or near UV-LED as the primary light source and the use of only one phosphor is given by Woan-Jen Yang, Liyang Luo, Teng-Ming Chen, and Niann-Shia Wang in Chem. Mater., 2005, 17 (15), 3883-3888. The authors describe an alumosilicate-based phosphor of the general formula CaAl2Si2O8: Eu2+, Mn2+, exhibiting a main emission peak centred at 425 nm and a broad emission band centred at 586 nm.
Another example of a white LED comprising a UV or near UV-LED primary light source and only one phosphor is given in US 2010/0259161 A, which discloses a co-activated phosphor based on the formula CaMg2Al6Si9O30. The main emission peaks of the described phosphor are centred at 467 nm and 627 nm, respectively. However, the emission ranges up to 800 nm, which is outside of the range most sensitive to the human eye.
Accordingly, there is still room for improvements and modern luminescent materials, preferably                exhibit high colour rendering indices,        exhibit at least two emission bands in the range of the VIS-light, preferably in the range of the VIS light which is in particular most sensitive to the human eye,        are effectively excitable by an UV or near UV emitting primary light source,        exhibit high quantum yields,        exhibit high efficiency over a prolonged period of use,        have high chemical stability, preferably against humidity or moisture        exhibit lower thermal quenching resistivity        are obtainable by method of production, which has to be cost efficient and especially suitable for a mass production process.        
In view of the cited prior art and the above-mentioned requirements on modern luminescent materials, there is still a considerable demand for alternative materials, which preferably do not exhibit the drawbacks of available phosphors of prior art or even if do so, to a less extend.